1) Field of the Invention
This invention relates to a linear bearing suitable for assembly in various mechanical apparatuses with a linear motion part, such as measuring instruments and machine tools, so that the linear motion part can be displaced by small force. This invention also relates to a cylindrical, resin-made retainer permitting easy formation by injection molding, ready assembly of balls therein, and fail-free holding of the balls so assembled.
2) Description of the Related Art
To reduce the force required for the displacement of a linear motion part, linear bearings with plural balls assembled therein have already been proposed, for example, as disclosed in Japanese Utility Model Publication (Kokoku) Nos. SHO 40-15222, SHO 53-42672 and SHO 61-36811 and Japanese Utility Model Application Laid-Open (Kokai) No. SHO 58-169220.
FIGS. 3-4 illustrate one example of such linear bearings known to date. An outer peripheral wall 2 of a shaft 1 as an inner race means and an inner peripheral wall 4 of an outer ring 3 are formed as cylindrical walls, respectively. Within a toroidal space 5 between the outer peripheral wall 2 of the shaft 1 and the inner peripheral wall 4 of the outer ring 3, a cylindrical retainer 6 is inserted for axial displacement, namely, displaceably in an axial direction (i.e., the horizontal direction in FIG. 3 and the direction perpendicular to the drawing sheet of FIG. 4). The retainer 6, which has been formed by injection molding a synthetic resin, press forming a metal plate or machining a copper alloy, retains plural balls 7 for rotation. When held in the retainer 6, these balls 7 are maintained in contact with the outer peripheral wall 2 of the shaft 1 and also with the inner peripheral wall 4 of the outer ring 3.
As the linear bearing is constructed as described above, a relative axial displacement between the shaft 1 and the outer race 3 can be effected by a small force owing to rolling of the plural balls 7.
In the conventional linear bearing constructed as described above, the plural balls 7 are arranged evenly in the axial direction in the retainer 6 so that the conventional construction involves problems as will be described next.
No problem arises as long as the shaft 1 and the outer race 3 undergoes a axial displacement relative to each other while they are maintained in parallel to each other. If a moment is applied between the shaft 1 and the outer race 3 in such a way that the shaft 1 and race 3 are rendered non-parallel to each other, balls 7 located at axially opposite end portions are strongly pressed against the peripheral walls 2,4, respectively, so that the bearing stress becomes extremely large at these end portions. This leads to such a problem that durability is extremely reduced compared to the application of loads in a direction parallel to the shaft 1 or, in an extreme case, impressions are formed in the peripheral walls 2,4 to prevent smooth rolling of the balls 7 and hence to result in the need for greater force for a relative displacement between the shaft 1 and the outer race 3.
With a view toward coping with such problems, Japanese Utility Model Publication (Kokoku) No. SHO 53-42672 proposes to progressively reduce the outer diameter of the balls 7 toward the axial ends. By making the outer diameter of the balls 7 different as proposed above, it is possible to avoid the application of large pressing force to the balls 7 located at both ends even when a moment is applied. This approach, however, has raised another problem as will be described next.
When the outer diameter of the balls 7 is made different in the axial direction of the retainer, the differences are extremely small so that they cannot be distinguished visually. It is also difficult to distinguish these differences by an automatic bearing assembly apparatus. Ultimate carefulness is therefore required upon their assembly, leading to an unavoidable increase in the manufacturing cost of the linear bearing.
Further, conventional resin-made retainers include, for example, the resin-made retainer disclosed in Japanese Utility Model Laid-Open (Kokai) No. SHO 49-136941 (first conventional example). This is a resin-made snap bearing retainer in which, in one of the inner and outer peripheral walls of a main body of the retainer, recesses reaching one end of the peripheral wall--ball pockets being open at said one end--are provided between the ball pockets to form thin--walled portions between the recesses and the openings of the adjacent ball pockets.
In addition, Japanese Utility Model Publication (Kokoku) No. SHO 53-5314 discloses a resin-made retainer, in which plural balls arranged for rotation between an outer cylinder and inner cylinder to absorb impact energy are fitted under pressure in ball retaining pockets of a retainer ring made of a synthetic resin and the retainer satisfies the following dimensional relationship: the inlet diameter of each ball holding pocket &lt; the diameter of each ball &lt; the maximum inner diameter of the holding pocket (second conventional example).
On the other hand, Japanese Patent Application Laid-Open (Kokai) No. SHO 48-14932 discloses (1) a metal-made ball retainer (third conventional example) and also (2) a metal-made ball retainer (second conventional example). The metal-made ball retainer (2) is obtained by injection-molding a thick-walled, cylindrical synthetic resin body with ball-shaped core pieces inserted therein, removing the core pieces before the cylindrical body cools, fitting metal balls in the resulting pockets, and then allowing the cylindrical body to cool so that the balls are retained by the resulting contraction. The metal-made ball retainer (2) is obtained by providing a metal plate with holding pockets for fitting balls therein, and then providing each holding pocket with a raised edge on one side of the metal plate--said one side becoming an inner wall when the metal plate is formed into a cylinder and said edge being tilted to define a diameter somewhat smaller than the diameter of the ball--and with another raised edge on the other side of the metal plate--said the other side becoming an outer wall when the metal plate is formed into the cylinder and said another edge being tilted to define a diameter somewhat greater than the diameter of the ball--so that, after the ball is fitted in the holding pocket from the outside, the raised edge on the outer wall is pressed to hold the ball in the holding pocket (fourth conventional example).
The first conventional example is, however, accompanied by the problem that, as the peripheral edge of each ball pocket in which the ball is held is thinner locally only at portions between the opening of the ball pocket and its adjacent recesses, the peripheral edge of the ball pocket cannot be easily deformed to a sufficient extent upon assembling the ball in the ball pocket and a large force is hence required for the assembly. It also involves the problem that difficulties are encountered upon pulling out the core pins, which were employed for the formation of the inner peripheral walls of the pockets upon injection molding, from the retainer so molded.
The second conventional example involves the problem that, as the inlet diameter of each ball holding pocket is made smaller than the diameter of the ball within the thickness of the cylindrical retainer made of the synthetic resin, the ball must be press-fit into the ball holding pocket and a large force is required for the assembly of the ball as in the first conventional example. Any attempt to overcome this problem by a reduction in the thickness of the retainer, however, results in the problem that difficulties arise in positive ball retention.
The third conventional example is accompanied by the problems that the molding process is complex and, due to the need for the removal of the core pins before the molded cylindrical body cools, the final product can hardly be provided with high dimensional accuracy, thereby making it difficult to hold the balls positively.
The fourth conventional example is made of a metal so that it can be formed more thinly than the synthetic-resin-made retainers of the first to third conventional examples. It is, however, accompanied by the problem that its production process is complex because of the need for the formation of the raised edges for the holding of the balls and also for the crimping of the raised edges after fitting the balls in the holding pockets.
The first to fourth conventional examples are accompanied by further problems. When a cylindrical retainer is employed in a linear bearing for an oil-immersed solenoid valve, the constructions of the first to third conventional examples all produce large flow resistance against oil which flows axially. The fourth conventional example is made of metal so that its weight is as much as about 6 times the weight of a resin-made retainer, thereby producing large inertia force. The fourth conventional example therefore has improperly poor response in axial movements.